Patent GB-683155-A (GB)
Title: Diffraction grating ruling engine
IPC: G02B5/18
Filed: 1950-05-09
Abstract: 683,155. Interferometers. FAIRWEATHER, H. G. C. (American Cyanamid Co.). May 9, 1950, No. 11531/50. Class 97 (i). [Also in Groups XIX and XL (b)] In a diffraction grating ruling engine wherein a ruling cutter is reciprocated across a blank; mounted on a carriage movable between cutter strokes by a lead screw, the carriage also traverses an optical member of an interferometer, illuminated by a narrow spectral line: having a Doppler broadening constant of at least 4000, to vary in length one of the light paths of the interferometer so that the resultant interference fringes are moved tranversely with carriage movement across an aperture of a mask covering a radiation detector transforming optical radiation into electric currents. Stepby-step relay means responsive to the detector output count the successive fringes, and start and stop a motor driving the lead screw to automatically displace the grating carriage between cuts by a spacing equal to half the light path difference corresponding to a predetermined whole number of interference fringes. In Fig. 1 a scribing carriage 1 reciprocated along parallel ways 2 by rod 3 bears a diamond cutter 5 operating on a blank mounted on a carriage 6 traversed along parallel ways 7 at right-angles to the cutter motion by a lead screw 8 driven through reduction gearing by a motor 9. A stationary framework 22 carries a Michelson interferometer illuminated by a line source of monochromatic light 11 free from fine structure (e.g. the green line, Š5460.74, of monisatopic Hg. 198) of which the Doppler broadening constant defined in the Specification is not less than 4000. The light is focused through a monochromator comprising the lenses 12, 14, a prism 13 and a slit 15, collimated by a lens 16, and falls on an inclined half-silvered mirror 17. Half the light passes through the reflecting coating, is totally reflected by the fixed mirror 20 and produces a virtual image on the back of the mirror 17, while the remainder is reflected from the surface of mirror 17, passes through the inclined compensating plate 23 of the same thickness and material as the mirror 17, and is totally reflected by the mirror 18 mounted on the carriage 6 at 90 degrees to the mirror 20, to produce a real image on the surface of mirror 17. The two images are recombined by a lens 24 on the mask 25, having a circular slit 26 (Fig. 3) and the resultant circular interference fringes expand outwardly across the mask and slit on movement of the carriage 6. Between each cut the motor 9 is energized by a known electronic step-by-step relay assembly 28 to rotate the lead screw traversing the carriage. Interference fringes cross the slit 26 with the movement, to successively illuminate the photocell 27. The resultant current pulses are counted by the relay assembly so that when a predetermined fringe is centred in the slit the motor 9 is stopped, and the next cut is made at a spacing of a number of half-wavelengths equal to the number of fringes counted. Successive cuts may thus be made at accurately known intervals in either direction from zero, if the mirrors 18, 20 are so positioned that zero path difference corresponds to a scriber position in the centre of the blank. The apparatus may be operated using the vertical fringes produced when mirrors 18, 20 are slightly displaced from right-angles, with a vertical slit, Fig. 4 (not shown). A Fabry-Perot interferometer may be employed, Fig. 2 (not shown), wherein monochromatic light from an illuminated slit is collimated to pass through two optical wedges, one of which is mounted on the carriage. The emergent light containing interference fringes is imaged on the mask of the photo-cell, and the apparatus functions as above, except that successive cuts may only be made in one direction from the position of zero path difference since at this point the two wedges are in contact. The red line, Š6438.47 of a cadmium arc, or the radiation Š5870.92 from isotopic krypton 84, may also be used as a light source, and the axes of the scriber and carriage movements may be disposed at angles other than 90 degrees for finer line spacing.